Watching this thriller horror movie

The film is about a man named Masuoka, played by Shinya Tsukamoto, who carries a camera everywhere he goes. He becomes obsessed with the idea of fear when he sees a frightened man shove a knife in his eye to commit suicide. Wishing to understand the fear that the dead man must have felt before his death, Masuoka descends into a labyrinthine underground area beneath the city, where he sees human-like creatures that walk on their hands and knees and whimper like dogs. While searching the series of tunnels and passages, Masuoka encounters a homeless underground inhabitant who warns him about the Deros. He then meets the ghost of Kuroki, the man who killed himself, and learns more about the underworld. After hours of searching, Masuoka discovers a mountain range with a village built by the underground dwellers. He finds a naked girl (played by Tomomi Miyashita) chained to the wall. He takes her back to his apartment and notices she doesn't eat, drink, or speak.
The girl, whom Masuoka dubs 'F', appears to be something other than human, and Masuoka becomes obsessed with understanding her. He sets up cameras that enable him to observe her from his cell phone when he leaves the apartment, and checks on her regularly. On a trip to the shopping mall he sees her speaking to someone off camera, and a menacing man in black appears behind him. When he returns to the apartment a woman in a yellow jacket is hiding in the stairway outside his door. Inside, he finds F convulsing, and attempts to feed her, but she will not eat. He discovers that twelve seconds of camera footage is missing, and receives a mysterious phone call from a pay phone warning him that he is in serious trouble.